Republican frontrunner Mitt Romney admitted he has made "mistakes" in his campaign as polls suggested he is at risk of losing the crucial Michigan primary on Tuesday to his main rival Rick Santorum.

Romney did not elaborate on what the mistakes had been other than to admit he had been hurt by gaffes relating to his image as being ultra-wealthy.

As voters flooded to the polling booths in Michigan, Romney showed signs of vulnerability.

Asked if he had been hurt by remarks he made over the last few days, specifically his throwaway line that he was not a particularly enthusiastic supporter of Nascar racing but he knew the team owners, he said bluntly: "Yes."

A few days earlier, talking about Detroit as the car capital of the US, Romney also provided more ammunition for those highlighting his wealth when he said his wife had two luxury Cadillacs.

He blamed himself rather than his campaign team. "I'm trying to do better and work harder and make sure that we get our message across," he said.

Romney used a rare press confernce to take a swipe at Santorum, describing him as "desperate" for resorting to what he called dirty tricks in the Michigan primary, where voters are flooding to the polls on Tuesday.

He was unusually coy about predicting the result and instead opted to say he would win the race in the long run. "In the final analysis, I anticipate being the nominee," Romney said.

He predicted a protracted struggle. "This is not going to be over in a day or two," he said.

Polls suggest the outcome is too close to call in a state that is crucial to the prospects of both men.

The Republican race would be thrown into chaos if Santorum, the dark horse at the start of the caucus and primary season, was to win.

It is a must-win state for Romney in the same way that he needed to win the Florida primary in January to block off a challenge from Newt Gingrich, who has since faded from the race.

Romney needs the victory in Michigan to stall Santorum but also to avoid the indignity of losing his home state.

Romney had the initial advantage of having a strong team that has managed to get out a lot of his support in early voting. But Santorum is enjoying a late surge and attracting large crowds to his rallies.

Santorum, in an effort to bridge the gap with Romney, has resorted to robocalls pleading with Democrats to come out to vote against Romney because he had opposed Barack Obama's 2008 bailout of the car industry on which Michigan depends.

Romney, at the press conference at his campaign headquarters in Livonia this morning, denounced Santorum for his appeal to Democrats and said he had failed to mention that he too had opposed the bailout. Romney described it as the act of "a desperate candidate". He added: "Republicans have to recognise there's a real effort to kidnap our primary process."

If Romney was to lose Michigan, the robocalls offer him an opportunity to claim he won among Republicans but lost because of mischief by Democrats, encouraged by Santorum.

Romney rarely gives press conferences and this was the first for three weeks, finally giving in to appeals from a frustrated media pack.

But polling day is a good day to have one, when it is just about too late for any gaffes to have an impact on the outcome of the primary.

Asked about the damage the infighting is doing to Republican chances in the November general election, a major concern in Republican ranks, Romney played it down, predicting it would be forgotten by then and people would be focused on other issues, such as the economy and maybe foreign policy.

And when asked about his inability to excite the Republican base, Romney made it clear he did not intend pandering to the party right any more than he has already.

"It is very easy to excite the base with incendiary comments," he said, a remark that might not play well with rightwingers, suggesting they are easily led. He went on to deliver the most memorable line from the press conference, aimed at Santorum and Gingrich. "We've seen throughout the campaign if you're willing to say really outrageous things that are accusative, attacking of President Obama, that you're going to jump up in the polls. I'm not willing to light my hair on fire to try and get support. I am who I am," he said.

The candidates are looking beyond Michigan to the 10 Super Tuesday states next week. Santorum is to visit Tennessee on Wednesday, where he is ahead in a state that Gingrich covets. He is also ahead in Ohio, the big prize of Super Tuesday.

Gingrich is ahead in his home state of Georgia.

Loss of these three states would be another setback for Romney, not compensated by his expected victories in Massachusetts, Vermont and Virginia, with neither Santorum nor Gingrich on the ballot. He needs to be seen to be beating Santorum in head-to-head contests, and that means Ohio.